Antibody-cytotoxic agent conjugates (or “antibody-drug conjugates (ADC)”) and cell binding agent-drug conjugates are emerging as a powerful class of anti-tumor agents with efficacy across a range of cancers. Cell binding agent-drug conjugates (such as ADCs) are commonly composed of three distinct elements: a cell-binding agent (e.g., an antibody); a linker; and a cytotoxic moiety. The cytotoxic drug moiety can be covalently attached to lysines on the antibody, resulting in conjugates that are heterogeneous mixtures of ADCs bearing varying numbers of drugs attached at different positions on the antibody molecule. Alternatively, the cytotoxic drug moiety can be covalently linked to cysteine thiol groups on the antibody through a thiol-reactive group, such as a maleimde group, to form site-specific ADCs. Conjugation reactions between the antibodies and the cytotoxic agents are often carried out in water or an aqueous solution with small amount of an organic solvent required for solubilizing the cytotoxic agents.
Benzodiazepine compounds, including tricyclic benzodiazepines, such as pyrrolobenzodiazepines (PBD), and tetracyclic benzodiazepines, such as indolinobenzodiazepines, have been employed as cytotoxic agents in linkage with antibodies to generate ADCs, which have shown promising antitumor activities. These benzodiazepine compounds contain imine bonds, which can bind to the minor groove of DNA and interfere with DNA function, resulting in cell death. Benzodiazepine compounds generally have very low solubility in water. To solubilize the benzodiazepine compounds in the conjugation reaction with antibodies, relatively large amount of organic solvent is required, which can de-stabilize the antibodies.
Therefore, there is a need to develop new methods for preparing conjugates of cell-binding agent and imine-containing benzodiazepine drugs.